Beam-induced redox chemistry in iron oxide nanoparticle dispersions at ESRF–EBS

With the increased brilliance at the European Research Facility–Extremely Brilliant Source (ESRF–EBS), a beam-induced reduction of non-stochiometric iron oxide nanoparticles (almost maghemite composition) to magnetite was observed in a mixture of ethanol and water with low ethanol concentration.

added. The reaction solution was then degassed under argon for about 2 hours and subsequently heated to 220°C with a heating rate of 130°C/h. The temperature was kept constant for 2 hours and then cooled to about 120°C. At this point the respective capping ligand dissolved in a DEG:H2O mixture was added (see Table S1). After the addition of the capping-agent solutions, the nanoparticle suspensions were stirred for another 10-15 minutes, while cooling to room temperature. When cooled to room temperature or the next day, the nanoparticle powder was precipitated by addition of ca. the double volume of acetone and isolated on a magnet. For purification, the powder was washed with absolute ethanol at least four times. To obtain the EtOH-H2O dispersions, after the last drying step, the nanoparticle powders was freshly re-dispersed by removing supernatant ethanol, drying at air and then adding water (10 mM HCl in case of ID15-A-2). This gives an aqueous dispersion with a little amount of residual EtOH. The concentration of the dispersions was determined gravimetrically (see Table S1). Dispersions in pure H2O were received by letting the powders dry completely (weakly covered with tin foil) after the last purification step, in the ventilated air of the fumehood, at least over night. The dry powder was then re-dispersed in water prior to the beamtime with a concentration of 10 g/L.

S2. Calculation of radiation dose
The radiation dose after 60 and 250 s (for ID31 and ID15-A) in kGy (Gray, 1 Gy = ), respectively, was calculated according to (Bondaz et al., 2020). With the approximation of the sample being water the mass of the sample complies to the irradiated volume. The beam absorption for the respective energies for water and a target length of 1 mm (diameter of capillary) was estimated to be 2% with an Radiation dose ID15-A before EBS-upgrade: The photon flux on the sample was estimated by the respective beamline scientist according to (Vaughan et al., 2020). The total flux focused on the sample with the used optics ('Transfocators and LLM') at ca 70 keV is now about 10 12 ph/s (Vaughan et al., 2020, see Figure 6) for full focussing, but ca. 2x higher for the focussing applied in the experiment. Before the EBS upgrade the flux was 20-40 times lower (Vaughan et al., 2020, see Figure 2) and therefore the flux was estimated to be between 5•10 10 -1•10 11 ph/s. Two limiting values for the radiation dose were calculated.

S3. Composition and crystal structure of IONPs
The IONPs are phase-pure (no wüstite side phase) and crystallize in inverse spinel structure. In Figure S1 panel a) it can be seen, that the reference pattern for maghemite in P43212 structure describes all observed reflexes of the IONPs. (Greaves, 1983) The utilized stoichiometry of the iron precursor salts in the synthesis should result in a Fe3O4 (magnetite) stoichiometry. Yet, since the IONP powders are stored in air and they are known to oxidize to ү-Fe2O3 (maghemite) over time, we expect a nonstochiometric composition in between Fe3O4 and ү-Fe2O3. (Cervellino et. al., 2014, Cooper et al., 2020 The presence of ordered vacancies (due to oxidation of Fe 2+ to Fe 3+ ) is affirmed by the presence (even though quite weakly above background level) of the three reflexes marked with *. Those are only present when the symmetry of cubic spinel structure is reduced to tetragonal by vacancy ordering on octahedral sites. (Greaves, 1983) For one IONP powder a PDF fit with P43212 structure (see Figure S1 panel b) was performed and the best fit was indeed found for an occupation of the Fe(4) site in between magnetite (1) and maghemite (0.33). The results of the performed fits are presented in Table S2.

S4. Evaluation of ethanol (EtOH) content
Since the IONPs were freshly re-dispersed in water directly after the purification, the dispersion contains a little amount of residual EtOH. The fact, that it is a low concentrated EtOH-H2O mixture can be recognized by the FSDP of the IONP dispersion of this mixture being shifted a little to lower Qvalues in comparison to water and IONPs in water, since pure EtOH exhibits its FSDP at ca. 1.53 Å -1 (see Figure S2 panel a)). Further, the FSDP also seems to broaden a bit. To track the EtOH amount down at ID31, a 6 vol% EtOH-H2O mixture was measured, too, and the diffraction patterns of IONP dispersion and this mixture match pretty well (see Figure S2 panel b)). Consequently, the amount of EtOH in the IONP dispersions is about 6 vol%. In order to show, that the EtOH amount is similarly low in all evaluated samples before and after the upgrade, the FSDP of the dispersions and water was fitted with a Gaussian fit on a linear background between 1.6-2.4 Å -1 (see Figure S2 panel c). The region for the fit wasn't chosen bigger in order to exclude the region of the Bragg peak of IONP dispersions at ca.
2.48 Å -1 . From Figure S2 panel a and b it is evident, that this region is sufficient to describe the shift caused by the little amount of contained EtOH. Compared to the position (and FWHM) of pure water, the FSDPs of dispersions are shifted to smaller Q values (and are broader), see Table S3.

S5. Gaussian fit to (440) reflex
In order to quantify the observed shift in relation to the scan number, for IONP dispersions in low concentrated EtOH-H2O solution at ID31 after the EBS upgrade, the (440) reflex in the Q-range between 4.10 -4.40 Å -1 of three scans (1,3 and 10) was fitted with a Gaussian on a background (see Figure S3). As background function a line was chosen. For comparison, the Gaussian fits were also performed for samples, where no shift of the reflexes was observed (IONPs in H2O at ID31, IONPs in EtOH-H2O at ID15-A before EBS upgrade). The results are presented in Table S4. The locations (loc) of (440) listed in Table S4 were taken to calculate the peak shifts in % presented in Table 1 according to ℎ = 10 − 1 10 • 100. The error on this shift was determined with a Gaussian error propagation.

Figure S3
Gaussian fit applied to the (440) reflex in the Q-region between 4.10 -4.40 Å -1 . The background was fitted with a linear slope.

S6. PDF modelling for lattice parameter
In order to show, that the shift to lower Q-values in I(Q), is a global phenomenon concerning the whole diffraction pattern and is indeed related to lattice expansion, the data was also evaluated in real space.
d-PDFs (difference pair distribution functions; dispersion data minus water background) were evaluated for three scans (1,3 and 10) of each sample. Figure S4a) illustrates that the distance correlations in these d-PDFs of an IONP dispersion in EtOH-H2O solution in the mid-r range are shifted to higher r with increasing scan number indicating lattice expansion. These d-PDFs were fitted in the range from 15-100 Å in order to quantify the lattice expansion (see Figure S4b) for one exemplary fit. Note, that the low r-range could not be fitted, since it contains signal from residual EtOH. For simplicity (no averaging of lattice parameters, in order to see lattice expansion) the cubic Fd 3 ̅ m model, in which magnetite/maghemite can also be described, was taken for the fitting. Fitted values were the unit cell parameter (a=b=c), a scale factor to match experimental and theoretical intensity, the spherical diameter, uiso of iron and oxygenisotropic atomic displacement parameters, as well as the oxygen position. In Table S5 the obtained lattice parameters as well as the goodness of fit parameters are listed for all evaluated samples. The lattice expansion (lattice exp) in % presented in Table 1 was calculated with the presented values in Table S5 according